As an undrafted rookie in 2001, he got a sense rather quickly that his running skills could translate.

But the other responsibilities, like blocking?

"I remember when I first got here and then I left and went back home for that little break, I told my family, 'I might be the dumbest football player ever in the world, because I do not know any of this,'" Rhodes said. "It's hard, it was overwhelming for me.

"Because I had never really been introduced to the game like that. It took me a little while and I finally started picking it up right before training camp and as I got in the games more and more and more and then by my second year it was pretty easy for me."

The Colts have a long history of plugging in rookie running backs and expecting them to accomplish what Rhodes identifies as priority No. 1: "All they care is if you are picking up the blocks and 18 isn't getting hit."

Helping protect No. 18 -- that's Peyton Manning, in case you're not a numbers person -- is a big part of an Indianapolis running back's job.

Now two explosive rookie running backs in the AFC South, Chris Johnson in Tennessee and Steve Slaton in Houston, have emerged as players their teams have a hard time taking out of the game on third down. But weapons that might get the ball on those plays first have to prove capable of protecting the quarterback.

If not ...

Well, Johnson's quarterback, Kerry Collins, suffered a broken jaw as a Carolina Panther when he got leveled by Denver linebacker Bill Romanowski on a blitz in a preseason game in 1997. The hit cost Romanowski a $20,000 fine. Colts president Bill Polian was running the Panthers then and cited the play as an example of the risk of a young running back in pass protection.

The guilty party, you might imagine, would have worn a Scarlett letter. But it turns out reporters, the team's PR chief and Collins don't remember precisely who it was and Polian didn't offer up the name. But the young back who would have most likely been in the field at that stage of a preseason game was second-year man Tim Biakabutuka, who missed all but four games of his rookie season and was still learning.

"Running backs are so much a part of the equation when it comes to protection," said Collins, who's been sacked just five times. "You look at all the great running backs -- Emmitt Smith, Marcus Allen -- those guys were phenomenal in pass protection. Of course, it gets overshadowed by all the things they could do with the ball. But they can really determine whether you are a good protecting team and we've got good ones here."

Colts coach Tony Dungy said the process is simple for a rookie running back. He can be an incredibly dangerous third-down weapon, but if he doesn't earn the trust of his position coach he'll find himself on the sideline on what may be the game's most important plays -- third down.

Most backs come into the league with qualities like speed, quickness, vision and power that mean they will be good runners, but have to learn a lot about blocking. Some cut blocked a lot in college, but NFL teams don't usually want them trying to topple an athletic NFL linebacker or safety in such a fashion. A lot of good runners were rarely asked to block in college. But not all of them come in as blank blocking slates.

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Titans head coach Jeff Fisher says his team is always trying to improve and isn't letting their 10-0 record distract them.

"I know in our case it benefitted us that Joseph Addai had played in that style of offense at LSU. He was familiar with a lot of that," Dungy said. "I would imagine that Slaton playing in a wide-open system and a lot of throwing at West Virginia was the same way. I think you do have to credit the coaches of the NFL teams, but a lot of those guys have experience coming out of college."

Johnson and Slaton are not liabilities in pass protection, but that doesn't mean they are always looking to block on third down, said former Titans running back Eddie George.

"You can't go out there and not understand the protection and who you have and the adjustments you have to make," George said. "But what you are seeing now, a lot of them aren't picking up the blitz, they are out on a hot route as a receiver. They're in maybe 80 percent of the time in the route, not in the blitz pickup."

Said Titans offensive coordinator Mike Heimerdinger: "If you don't want him protecting, then you can scheme it up so he doesn't have to protect, then you get him out in the flat. You can do both. You can keep a tight end in and let him go free release. However, you've got to get a guy who can change the game on the field, you've got to do that. If they can't protect you've got to change you're scheme up and get them out [on a quick route]. Chris can do both, we've used him in both all the way."

So if Collins sees a free rusher coming at him, he should also be able to see Johnson leaking out to a spot where he can be hit with a quick pass -- the release of which should limit the chances of absorbing a shot.

Johnson's a quality pass catcher who's being asked to do more as a receiver. With 30 catches, he's second on the Titans. But he rates himself as an adequate, super-willing blocker. He said he blocked plenty at East Carolina, but that the schemes there were nowhere near as sophisticated as the ones he is asked to understand now.

His position coach, Earnest Byner, tells him it's usually about desire.

"He let me know it's all about wanting to. It doesn't matter how bad your technique is, if you want to block then you can do it," Johnson said. "My technique is OK. I can get better. But the want to is perfect."